In view of global environmental issues, biologically derived polyesters have been gaining attention as biodegradable plastics that can be readily degraded in nature or as “green” plastics that can be synthesized from renewable carbon resources such as sugars or plant oils. In particular, polylactates can be obtained at relatively low cost. The melting points thereof are 170° C. or higher. Therefore, polylactates have sufficient heat resistance and thus can be shaped by melt molding. Accordingly, polylactates have been expected to be used as highly practical biodegradable polymers.
However, polylactate production has been conventionally carried out by neutralizing and purifying lactic acid produced by microorganisms, forming a dimeric cyclic compound (lactide), and carrying out polymerization as described in JP Patent Publication (Kokai) No. 2004-204464 A, which is problematic in terms of cost.
Hitherto, many microorganisms have been reported to have the ability to produce a polyester using a sugar as a carbon source (Non-Patent Document 1). A representative example of a biodegradable plastic produced by a microorganism is poly-3-hydroxybutyrate (polyhydroroxybutyrate (PHB)) consisting of 3-hydroxybutyrate (3βB) monomers. PHB is a thermoplastic polymer having a melting temperature of approximately 180° C. PHB has excellent melting processability as well as biodegradability, which is advantageous. At the same time, PHB is hard and fragile because of its high crystallinity. That is to say, it has poor shock resistance, which is problematic in terms of physical properties.
As a method for solving problems regarding physical properties of PHB, a method for producing a polyester copolymer consisting of 3HB and a different hydroxyalkanoate using a microorganism has been developed.
For example, Patent Document 1 discloses a method for producing a copolymer consisting of 3HB and 3-hydroxyvalerate (3HV). In addition, Patent Document 2 discloses a method for producing a copolymer consisting of 3HB and 3HV by allowing a microorganism of the genus Methylobacterium (Methylobacterium sp.), Paracoccus (Paracoccus sp.), Alcaligenes (Alcaligenes sp.), or Pseudomonas (Pseudomonas sp.) to come into contact with a primary alcohol having a carbon number of 3 to 7.
Such copolymer consisting of 3HB and 3HV has a higher degree of flexibility than PHB. In addition, it has been confirmed that an increase in the 3HV content in a polyester copolymer results in enhanced flexibility. In the above method for producing a copolymer consisting of 3HB and 3HV using a microorganism, the 3HV content in a polyester copolymer is regulated by adding, for example, propionic acid in the case of Patent Document 1 or propane-1-ol in the case of Patent Document 3 to a medium.
For instance, P(3HB-co-3HH), which is a two-component polyester copolymer consisting of 3HB and 3-hydroxyhexanoate (hereinafter abbreviated as “3HH”), and a method for producing the same are disclosed in Patent Documents 4 and 5. In the methods for producing a P(3HB-co-3HH) copolymer disclosed in these Patent Documents, fermentative production from a fatty acid such as oleic acid or fat and oil such as olive oil is carried out using Aeromonas caviae isolated from the soil. In addition, it has been reported that a recombinant strain is obtained by cloning the PHA synthase gene from A. caviae and introducing the cloned gene into Alcaligenes eutrophus for production of P(3HB-co-3HH) with the use of fatty acid as a carbon source and the recombinant strain (Patent Document 6).
In addition, in any case of the above methods for producing a polyester copolymer using a microorganism, it is necessary to use a polyhydroxyalkanoate synthase, which is an enzyme protein having activity of directly synthesizing a polymer. In addition, it has been attempted to modify such synthase so as to control the monomer unit mole fraction. For example, Patent Document 7 discloses a mutant enzyme capable of producing PHB with a high 3HB content, which is obtained by modifying the amino acid sequence of a polyhydroxyalkanoate synthase of a microorganism that has been identified as Pseudomonas sp. 61-3.
Meanwhile, a polyester copolymer consisting of a non-3-hydroxyalkanoate component serving as a monomer unit is expected to have physical properties differing from those of the above polyester copolymer. Patent Document 8 discloses a method for producing, as an example of such polyester copolymer comprising a non-3-hydroxyalkanoate component serving as a monomer unit, a polyester copolymer consisting of 3HB and lactate (LA) by culturing Ralstonia eutropha (previous name: Alcaligenes eutrophus) incorporating the nucleic acid encoding propionyl-CoA transferase of Clostridium propionicum with the addition of lactate to a medium. The document further discloses a method for producing a copolymer consisting of 3-hydroxyhexanoate, 3-hydroxyoctanoate, 3-hydroxydecanoate, and lactate by culturing Escherichia coli incorporating the nucleic acid encoding C. propionium-derived propionyl-CoA transferase and the nucleic acid encoding Pseudomonas sp. 61-3-derived polyhydroxyalkanoate synthase with addition of lactate and decenoic acid to a medium.
In the above, a polyester copolymer consisting of 3-hydroxyalkanoate serving as a monomer unit, a polyester copolymer consisting of a non-3-hydroxyalkanoate component serving as a monomer unit, and a method for producing the same using a microorganism are described.
However, there have not been any reports of methods for efficiently producing polylactate via microbial fermentation with the use of sugar as a starting material.    Non-Patent Document 1: “Biodegradable Plastic Handbook,” Biodegradable Plastics Society, 1995, pp. 178-197, (published by NTS Inc.)    Patent Document 1: JP Patent Publication (Kokai) No. 57-150393 A (1982)    Patent Document 2: JP Patent Publication (Kokai) No. 5-74492 A (1993)    Patent Document 3: JP Patent Publication (Kokoku) No. 7-79705 B (1995)    Patent Document 4: JP Patent Publication (Kokai) No. 5-93049 A (1993)    Patent Document 5: JP Patent Publication (Kokai) No. 7-265065 A (1995)    Patent Document 6: JP Patent Publication (Kokai) No. 10-108682 A (1998)    Patent Document 7: WO2003/100055    Patent Document 8: WO2006/126796